Rotary shredders contain a rotating rotor. Beater or flail mallets are pivoted to this rotating rotor. The beater or flail mallets are pivotable around axes which are in parallel with the axis of rotation of the rotor. During a rotation of the rotor the beater or flail mallets are pulled outward by the centrifugal force. Then they hit material to be comminuted or shredded like a hammer while rotating. For example, such rotary shredders are used for comminuting waste wood. If a piece of material is not comminuted by a beater or flail mallet immediately, the beater or flail mallet mounted pivotable to the rotor draws back after the hit. Many beater or flail mallets provide a cutting edge trying to chop the material to be comminuted according to the manner of an axe or a hoe.
In the operation of the rotary shredder the beater or flail mallets are exposed to extensive strain and are subject to heavy wear. Generally the beater or flail mallets are wearing parts. It is known to form beater or flail mallet in two parts. A base body with considerable mass is mounted pivotable to the rotor with a pivoting bearing. A cutting head with a cutting edge striking the materials to be comminuted is attached to this base body. Nevertheless, the cutting head is subject to wear and must be replaced every so often. However, the base body does not normally need to be replaced. The cutting head, or at least the cutting edge, can be hardened. Important in doing so is to produce the attachment of the cutting head attached to the base body in such a manner able to absorb the heavy beating loads and not provide any weak points. The beating forces should be transferred from the base body (with large mass) to the cutting head and therewith to the materials to be comminuted completely. The reaction force during the impact on the materials to be comminuted should be absorbed by the base body.
Different beater or flail mallets of this manner are known.
U.S. Pat. No. 2,267,865 discloses a L-shaped mallet for a rotary shredder in which a first arm supports a pivot bearing on its end and a crushing body forms the second arm of the “L”. Generally this crushing body is cubical and provides pairs of cutting edges on opposite side surfaces. The cutting edges extend in parallel with the axis of rotation of the L-shaped mallet. This block-like crushing body is screwed to the first arm of the mallet with a threaded bolt. Thereby the threaded bolt extends crosswise to the cutting edges along the plan of symmetry between the side surfaces forming the cutting edges. The cutting edges are formed by almost 90° edges of the cubical crushing body. A rectangular projection is seated on the first arm to which the cubical crushing body is attached with a corresponding rectangular passage channel. The threaded bolt is passed through a bore in the projection and the passage channel. In this arrangement the forward side surface of the cubical crushing body basically is perpendicular to the moving direction. During rotation of the rotor the crushing body hammer-like strikes the materials to be comminuted with its side surface forming the cutting edges.
The German patent document DE 29 43 456 A1 points out a two-part beater or flail mallet for rotary shredders. In this beater or flail mallet the cutting edges of the cutting head are aligned with the end faces of the base body. The attachment of the cutting head to the throughout plane surface of the base body is carried out with only one screw.
U.S. Pat. No. 5,285,974 teaches a two-part beater or flail mallet for a rotary shredder. In this beater or flail mallet a comminuting body is screwed to a base body. The base body has a block-like main part with a pivot bearing and a flat part projecting from it with substantial less thickness. A half-moon-like comminuting body with two protruding cutting edges is screwed to this flat part. The comminuting body is vibration-isolated from the base body. The cutting edges of the comminuting body are extending over the base surface of the flat part in direction of motion of the beater or flail mallet.
The published US patent application US 2004/0056129 A1 shows a two-part beater or flail mallet for a rotary shredder with a base body and a comminuting head or cutting head. The base body provides a section of larger thickness forming a pivot bearing at its end. With this pivot bearing the beater or flail mallet is pivoted to a rotor. Outwards, thus away from the pivot bearing the section of larger thickness is followed by a section of less thickness forming a basically plane and about radial to the axis of rotation extending support surface on its front side. Between the support surface and the surface of the section of larger thickness a step is provided with an inclined area. A cutting head with an approximately trapezoidal cross section is seated on the support surface. Cutting edges are provided by the acute angles of the trapezoidal cross section of the cutting head. Thereby the cutting head is seated on the support surface with its back surface corresponding to the smaller edge of the trapeze. The cutting edges are provided on the surface opposite to the support surface. The radial outer cutting edge protrudes inclined upwards and outwards beyond the end face of the base body. The other cutting edge is supported by the inclined area of the step. On the one hand the connection between the base body and the cutting head is carried out by the cutting head fitting against the support surface and the inclined area of the step as well as a threaded bolt. On the other hand a truncated pyramidal projection is provided on the support surface. The cutting head provides a truncated pyramidal recess complementary to the projection. The cutting head is attached with its recess on the projection. The threaded bolt extends through the recess and the projection. Thereby, opposite side surfaces of the pyramid are aligned with the cutting edges.
In this arrangement very strong forces and moments must be transferred from the cutting head to the base body, when the cutting edge lying far outwards beyond the base body strikes a part to be comminuted with a large impulse. In the beater or flail mallet according to US 2004/0056129 A1 the cutting head is weakened just at the point transferring substantially the forces and moments to the base body by the truncated pyramidal recess and the thereto aligned recess for the head of the bolt. This can lead to a breakage of the cutting head during continuous operation limiting the lifetime of the beater or flail mallet.